5 notebooks cost $9.05. Which equation would help determine the cost of 13 notebooks?
There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 13 notebooks. Since 13 notebooks cost $x$ , we have the following proportion: $\dfrac{13}{x}$ We can write the fact that 5 notebooks cost $9.05 as a proportion: $\dfrac{5}{\$9.05}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{13}{x} = \dfrac{5}{\$9.05}$